hwpmc(4): update DEBUGGING section details

[mhorne]

Summary

The examples I wrote previously do not work. We parse the flags using
explicit names, not the shortened flag suffix. Fix the list of major
flags, and add a compact list of minor flags. Fix the examples, and
tweak some wording for clarity.

Fixes: 5fc97cc32553 ("hwpmc(4): document debugging options")

Test Plan

Build a kernel with PMC_DEBUG and try setting the sysctl with the examples provided.

[mhorne]

share/man/man4/hwpmc.4

585–620

FYI this renders as:

The minor flags for each major flag group can vary.  The individual minor
flag names are:

      allocaterow, allocate, attach, bind, config, exec, exit, find,
      flush, fork, getbuf, hook, init, intr, linktarget, mayberemove,
      ops, read, register, release, remove, sample, scheduleio, select,
      signal, swi, swo, start, stop, syscall, unlinktarget, write